During the coining process of money, die rotation can, and often does, occur, resulting in the orientation of the reverse face (tail) of the coin to be rotated somewhat from the orientation of the obverse (head) of the coin. In such coining process, the dies used are hardened steel tools which impress the desired image on a coin blank. The dies are used in pairs, coining the obverse face and the reverse face of a coin simultaneously, in a machine referred to as a stamping press. By design, the obverse face and reverse face are intended to be 180.degree. out of phase. However, one or both of the dies may become loose in its fixture and rotate, causing the image on one side of the coin to be offset from the theoretical or "perfect" 180.degree. obverse-to-reverse relationship. This offset is called die rotation error.
In the numismatic industry (coin collecting), the value of a given coin is increased in proportion to the degree of its die rotation error, the frequency (or lack thereof) of such error in a given issue, and the rarity of the coin itself. At present, there are no methods known to measure die rotation error other than to place the coin on a circular scale, mark the edge of the coin, and then turn the coin over keeping the mark at the same position on the scale and estimate the amount of rotation. Alternatively, an elaborate and cumbersome system of mirrors and scales, arranged so that one may simultaneously see both sides of the coin, may be used. However, numismatists do not like to mark their valuable coins, and mirror and scale devices are unwieldy and inconvenient.
Certain die rotation errors have been cataloged. For example, "The Major Variety and Oddity Guide to United States Coins", 5th Edition, Library of Congress Catalog Card No. 67-18968, lists the following:
1966 Nickel, rotated 15.degree., 30.degree. 60.degree., 70.degree., 90.degree. PA1 1960 Dime, rotated 20.degree. PA1 1930P Nickel, rotated 60.degree. PA1 1926D Cent, rotated 345.degree. PA1 1924P Cent, rotated 10.degree., 35.degree., 45.degree., 60.degree., 180.degree., 240.degree. PA1 1798 Large cent, rotated 90.degree. PA1 1809 Half cent, rotated 160.degree. PA1 1816 Cent, rotated 310.degree.
Certain earlier-dated coins possessed the following rotation errors:
There is provided according to this invention a device for quickly, accurately and conveniently measuring the die rotation error for any given coin without marking or disturbing the coin in any way. The device is compact, easily portable, and may be carried in one's pocket such that a numismatist visiting a coin show may obtain on-the-spot measurements of rotation error of coins he may wish to add to his collection.